Cogeneration is the simultaneous production of both useful thermal energy (usually steam) and electrical energy from one source of fuel. One or more gas turbines followed by a waste heat boiler using natural gas as fuel for both the turbines and to heat the exhaust gases from the turbines represent a typical system.
In recent years, the cogeneration market has expanded rapidly due in part to the Public Utility Regulatory Policy Act of 1978. PURPA gave financial incentive to cogenerators that sell excess electrical power and requires that utilities purchase power from cogenerators. It also allows utilities to own up to 50% of a cogeneration facility and receive the benefits of this status.
One problem with this system is the level of NOX emissions generated with the combined firing cycle. Cogeneration plants using conventional gas turbines and auxiliary fuel fired heat recovery boilers to produce electricity and steam are being subjected to stringent NO.sub.X emission standards requiring levels below the 150 ppm range. New Source Performance Standards (NSPS) strictly limit NOX emission. To meet the regulations for NOX emissions, several methods of NOX control have been employed. These can be classified as either equipment modification or injection methods. Injection methods include injection of either water or steam into the combustion zone to lower the flame temperature and retard the formation of NOX, since the amount of NOX formed generally increases with increasing temperatures, or injection of ammonia to selectively reduce NOX. Water or steam injection, however, adversely affects the overall fuel efficiency of the process as energy is absorbed to vaporize the water or heat the steam that otherwise would go toward heating the turbine gas and be ultimately converted into usable electricity or steam. A process involving the injection of ammonia into the products of combustion is shown, for example in Welty, U.S. Pat. No. 4,164,546. Examples of processes utilizing ammonia injection and a reducing catalyst are disclosed in Sakari et al, U.S. Pat. No. 4,106,286; and Haeflich, U.S. Pat. No. 4,572,110. Selective reduction methods using ammonia injection are expensive and somewhat difficult to control. Thus, these methods have the inherent problem of requiring that the ammonia injection be carefully controlled so as not to inject too much and create a possible emission problem by emitting excess levels of ammonia. In addition the temperature necessary for the reduction of the oxides of nitrogen must be carefully controlled to get the required reaction rates.
Equipment modifications include modifications to the burner or firebox to reduce the formation of NOX. Although these methods do reduce the level of NOX, each has its own drawbacks. Combustion equipment modification affects the performance of the turbines and limits the range of operation. Moreover, cogeneration plants of this type employed for generating electric power and steam are being subjected to increasingly stringent NOX emission standards, and a satisfactory emission control system is required to minimize the undesirable emissions exhausted to the atmosphere. A selective catalytic reduction system is presently considered by some authorities to be the best available control technology for the reduction of NOX from the exhaust gas of a cogeneration plant, and as a consequence is required equipment. Currently available selective catalytic reduction systems used for the reduction of NOX employ ammonia injection into the exhaust gas stream for reaction with the NOX in the presence of a catalyst to produce nitrogen and water vapor. Such systems typically have an efficiency of 80-90 percent when the exhaust gas stream is at temperature within a temperature range of approximately 600.degree.-700.degree. F. The NOX reduction efficiency of the system will be significantly less if the temperature is outside the stated temperature range and the catalyst may be damaged at higher temperatures.
The engine outlet temperature of most gas turbine cogeneration plants, at full or rated load of the gas turbine engine, is conventionally between approximately 775.degree. F. and 1050.degree. F. Since the exhaust gas temperature is above the optimum temperature range of the usual selective catalytic reduction system, it is necessary to reduce the temperature of the exhaust gas stream before it passes through the system. Current practice is to provide steam superheater and/or steam generating tubes upstream of the system to withdraw heat from the exhaust gas stream to cool the gas to a preselected desired nominal temperature before it passes through the system. This imposes various operating limitations on the cogeneration plant which either seriously limit the operating range of the gas turbine engine or require an undesirable exhaust gas bypass or other mechanism for diverting a portion of the exhaust gas stream. Where supplementary firing is provided to increase steam production, the supplementary firing is conventionally located downstream of the selective catalytic reduction system because it heats the exhaust gas above the optimum temperature range of the system. That is undesirable because it reduces the steam generating efficiency and produces additional NOX that is not removed by the system
It is therefore an object of the present invention to provide a cogeneration system of the type described wherein the level of NOX in the emissions is lowered in an improved manner.
It is another object of the invention to provide a cogeneration system wherein NOX emissions are controlled without adversely affecting the operation of the turbine.
It is a further object of the invention to provide a cogeneration system embodying a gas turbine wherein NOX emissions are reduced without adversely affecting the fuel efficiency of the system.
It is a still further object of the invention to provide for NOX removal in a cogeneration system employing a gas turbine which is more economical and more readily controlled than systems heretofore commonly employed in the cogeneration art.